1. Field of the Invention
The present invention relates to an optical transmitter device and an optical transmitter module.
2. Description of the Related Art
Optical modules with a semiconductor optical modulator device, or an optical modulator-integrated semiconductor laser chip, built in are used as one of the key devices of a transceiver for middle-to-long-distance optical fiber transmission. Optical transceivers have been increased in speed as broadband networks become popular, and now high transmission rate optical transceivers having a bit rate of 10 Gbits/s or higher are widely employed. Optical modules suitable for use in such optical transceivers are demanded to be small in size and low in power consumption, in addition to achieving an excellent optical transmission wavelength quality.
Electro-absorption (EA) optical modulator devices are often used as optical modulator devices which fit for size reduction. Electro-absorption optical modulator devices are driven with the output voltage waveform of a signal output circuit (e.g., driver IC). An impedance of the electro-absorption optical modulator device varies greatly depending on the ON/OFF state of an optical signal. Obtaining a favorable waveform by merely driving the electro-absorption optical modulator device alone is therefore difficult due to an impedance inconsistency. Many conventional optical modules obtain an excellent optical transmission waveform by placing a resistor device of approximately 50 Ohms close to an electro-absorption optical modulator device and by using the resistor device as a termination resistor of the signal output circuit, with one end of the resistor device connected to the optical modulator device and another end of the resistor device connected to a ground potential. However, in this type of optical modules having the termination circuit structure described above, the need to apply a bias voltage to the electro-absorption optical modulator device causes an originally unnecessary direct current in the resistor device, which can lead to inessential power consumption.
As a conventional technology of reducing power consumption, a structure has been proposed which places a termination resistor circuit with a resistor device and a single DC blocking capacitor device, or a plurality of parallelly connected DC blocking capacitor devices, connected in series close to an optical modulator device. An example of this structure is disclosed in JP 06-230328 A, where a termination resistor circuit made up of a chip resistor, a multi-layered ceramic capacitor, and a parallel-plate capacitor is placed close to an electro-absorption optical modulator device. In the termination resistor circuit, a multi-layered ceramic capacitor having a capacitance value of 0.01 microfarads and a parallel-plate capacitor having a capacitance value of 100 pF are connected in parallel, and the parallelly connected capacitors are connected in series to a chip resistor having a resistance value of 50 Ohms. JP 06-230328 A uses the parallel-plate capacitor which has excellent high-frequency characteristics to keep the impedance of the termination resistor circuit in a high frequency range at 50 Ohms, and uses the multi-layered ceramic capacitor which has excellent capability in increasing the capacity to keep the impedance of the termination resistor circuit in a low frequency range (frequency range above 300 kHz, which is a lower cutoff frequency) at 50 Ohms, thereby constructing a broadband termination resistor circuit, and balancing low power consumption and an excellent optical transmission waveform.
However, as optical transceivers become smaller in size in accordance with a standard called Multi Source Agreement (MSA), a problem has arisen in that the use of the conventional technology described above in building an optical module hinders size reduction. In order to meet SFP+, which is an MSA standard for 10-gigabit Ethernet (registered trademark) optical transceivers, an optical module on the transmission side (Transmitter Optical Sub Assembly: TOSA) needs to be implemented with the use of a 5.6 mm TO-CAN package, which is smaller in size than conventional ones. With the technology described in JP 06-230328 A, where a multi-layered ceramic capacitor having a capacitance value of 0.01 microfarads or more needs to be installed in an optical module, there is no space left for the multi-layered ceramic capacitor once an optical modulator-integrated semiconductor laser chip, a resistor device, and other components indispensable to the optical module are installed in a TO-CAN package that is 5.6 mm in diameter. Manufacturing a small-sized optical module with this technology is therefore difficult.